This invention relates to an unsaturated polyester resin composition and, more particularly, to an unsaturated polyester resin composition containing an unsaturated polyester, a vinyl monomer, a maleimide compound and a hindered phenolic compound and/or carbon.
The unsaturated polyester resin is extensively used in the form of fiber reinforced plastics by being processed by a method consisting usually in mixing glass fibers and/or inorganic fillers to a resin and heating or curing by curing agents at ambient temperature. More specifically, automotive parts or electrical components are produced by a curing method such as a sheet molding compound method (SMC method) or a bulk molding compound method (BMC method), while fishing boats, sewage purifiers or pool walling members are manufactured by an ambient temperature curing method, such as a hand lay-up method or a spray-up method.
However, the polyester resin composition has a deficiency that it is generally poor in chronological stability and tends to be gelled in a shorter time when preserved during summer. Although it may be contemplated to add ordinary polymerization inhibitors or polymerization retarders in the composition or to store the composition under cooling, it has been found that these measures are hardly effective in eliminating the above mentioned deficiency.
On the other hand, it is also well-known to add carbon black to the unsaturated polyester resin to provide the resin with electrical conductivity or anti-static properties or to process the resin for improving its dyeing properties or strength.
However, the unsaturated polyester resin admixed with carbon black has a deficiency that it can be cured only insufficiently or with delay, such that the curing cycle cannot be improved and, in addition, the surface conditions of the cured molded articles are deteriorated.
In order to remove this deficiency, it has been proposed to add surface-inactivated carbon black selectively to the resin or to use an initiator which is hardly affected by carbon black. However, none of these measures is effective to provide fully satisfactory physical properties.